WARSAW, Poland — Denouncing Russia’s actions in Crimea as “nothing more than a land grab,” Vice President Joe Biden warned Russia on Tuesday that the U.S. and Europe will impose further sanctions as Moscow moved to annex part of Ukraine.

With limited options, the United States was seeking ways to show it won’t stand idly by as Russian President Vladimir Putin signed a treaty for the Ukrainian peninsula of Crimea to join Russia. So far, Putin has been undeterred by sanctions and visa bans levied by the U.S. and the European Union, and there’s no U.S. appetite for military intervention.

“Russia has offered a variety of arguments to justify what is nothing more than a land grab, including what he said today,” Biden said in Poland, which shares a border with both Russia and Ukraine. “But the world has seen through Russia’s actions and has rejected the flawed logic behind those actions.”

Biden arrived early Tuesday in a region on edge over Russia’s nascent aggression in Crimea. Amid eerie echoes of the Cold War, U.S. allies including Poland have raised concerns that they could be next should the global community be unable to persuade Putin to back down.

Former Soviet states are among the most alarmed by the prospect that Moscow could be resuming its traditional imperial ambitions. But Ukraine is at greater risk militarily because it lacks membership in NATO and the promise of collective defensive measures that NATO membership provides.

In a clear warning to Moscow not to test other nations along its border, Biden said the U.S. commitment to defending its NATO allies is “ironclad.” He promised more sanctions would be coming, along with new NATO training and exercises that will take place in Poland.

The vice president said the U.S. was considering rotating American forces to the Baltic region as a step toward ensuring the collective defense of NATO allies. Those forces could conduct ground and naval exercises, plus engage in training missions.

Meanwhile, major Western powers sought fresh ways to show that Russia would incur real costs unless it changes course.

The White House announced that President Barack Obama was inviting the leaders of the G-7 group of nations to a meeting in Europe next week to discuss further action. The group normally meets under the banner of the G-8, including Russia, but has suspended preparations for upcoming G-8 talks.

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And in London, British Foreign Secretary William Hague says the U.K. was suspending military cooperation with Russia in light of the crisis.

“It’s a simple fact that Russia’s political and economic isolation will only increase if it continues down this dark path,” Biden said, adding that virtually the entire world rejects the referendum in Crimea on Sunday that cleared the way for Russia to absorb it.

For his part, Putin seemed to shrug off the tough talk from the West, describing Russia’s move to add Crimea to its map as correcting past injustices. In an emotional, live speech from the Kremlin, he said that “in people’s hearts and minds, Crimea has always been an integral part of Russia.”

Russia’s move in clear defiance of its neighbors and the U.S. ups the pressure on Biden to convince its NATO allies that the U.S. won’t succumb to Russia’s aggressive moves.

In sessions Tuesday in the Polish capital and later in Lithuania’s capital, Vilnius, Biden was to discuss the crisis with the leaders of Estonia, Lithuania and Latvia — three Baltic nations that are deeply concerned about what Russia’s military intervention in Ukraine’s Crimean Peninsula might portend for the region.

All four countries share borders with Russia, while Poland also borders Ukraine. Poland broke away from Moscow’s domination in 1989 and was a vocal advocate for Ukraine forging closer ties with the E.U. — a dispute at the heart of Ukraine’s political crisis.

“This trial, this challenge that we are facing will not be for a month or a year,” Tusk said after meeting with Biden. “We are facing a strategic perspective for many years to come.”

Biden said the goal is for NATO to emerge from this crisis stronger and more unified than ever. While in Europe, Biden planned to discuss what additional steps the U.S. can take to shore up security for Poland and the Baltics, such as increased training, said a senior administration official, who wasn’t authorized to comment by name and demanded anonymity.

At Warsaw’s request, the U.S. last week sent some 300 air troops and a dozen F-16 fighters to Poland for joint training in a show of military support for a key ally.

Also on the agenda: long-term energy security in Europe, a key factor that has confounded the West’s attempts to display a united front in punishing Russia. Much of Europe is heavily dependent on Russian natural gas, and European countries have major economic interests in Russia that could be in jeopardy if Moscow retaliates with sanctions of its own.

Republican lawmakers and a handful of European countries, including Poland, have urged the White House to accelerate approval of U.S. natural gas exports, but the White House has insisted that would take too long and says Russia is too dependent on gas revenues to cut off Europe.

One option that apparently isn’t on the table: rethinking the U.S. posture on missile defense in the region. Poland is still displeased about Obama’s 2009 decision to cancel the final phase of a defense system sorely Poland wanted as a hedge against Russian missiles. Biden said the smaller, phased-in system Obama chose instead is on schedule for completion.

Associated Press writers Monika Scislowska in Warsaw and Nedra Pickler in Washington contributed to this report.